The present invention relates to a hydraulic control method for a hydraulically-operated load device, and more particularly, to a hydraulic control method conducive to the improvement of the hydraulic responsiveness of electrical/hydraulic converters, such as duty-controlled solenoid valves, used in an automatic transmission.
Duty-controlled solenoid valves are widely used as electrical/hydraulic converters in various hydraulic control circuits which include, for example, transmission control clutches and brakes of vehicular automatic transmissions. These solenoid valves serve to keep constant the ratio (duty factor) between the pressurized oil introduction and discharge periods of an oil passage system, which connects with the clutches or other hydraulically-operated load devices, thereby setting the time-based average pressure of the oil passage system, i.e., the operating oil pressure supplied to the load devices, to a desired pressure level corresponding to the constant duty factor. Conventionally, there have been the following problems to be solved in varying the duty factor of one such solenoid valve to change the oil pressure of the oil passage system from a first value to a second value.
If the duty factor of the solenoid valve is directly changed from a first duty factor, corresponding to the first pressure value, to a second duty factor, corresponding to the second pressure value, in order to change the oil pressure from the first value to the second, the oil pressure inside the piping system cannot be increased following such a change of the duty factor, on account of the resiliency of the piping system. Moreover, the solenoid valve is repeatedly turned on and off with the set duty factor. When increasing the oil pressure inside the oil passage system, therefore, the discharge period for the pressurized oil becomes an idle period. When lowering the oil pressure, on the other hand, the oil introduction period is rendered idle. Owing to this principle of control, it takes much time to attain the oil pressure corresponding to the specified duty factor (second duty factor). This electrical/hydraulic response delay, as it is called, may sometimes raise problems on the control of clutch operation and the like.